Remove blackhole kvstore (unused)

Change-Id: I3a3963edad727b87e57cce9f46cc6d1886cc5f14
Reviewed-on: http://review.couchbase.org/26992
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>